1. Field of the Invention
This invention relates to a differential voltage amplifier capable of amplifying signals having an input common-mode voltage which can be lower than the lower supply voltage or the substrate in the case of an integrated circuit amplifier embodiment.
2. Brief Description of the Prior Art
In many load control systems, monitoring of load current is required as a means of providing feedback to a control loop. For example, in a motor drive system where the motor is the load, control of the load current (current passing through the motor) is required to control motor torque. Typically, load current is monitored by passing such current through a low value resistor of a value usually less than 1 ohm with an amplifer across the resistor to amplify the sensed current to a more usable voltage range. The amplified voltage may then be digitized by an A/D converter and used within a control algorithm or it can be connected to the input of a comparator in a real-time, cycle-by-cycle control loop, etc. A system of this type is shown in FIG. 1 wherein the motor load 1 is coupled via an H-bridge circuit composed of transistors 3, 5, 7 and 9 to the low value sense resistor 11 and then to ground through a voltage offset 13 of up to about .+-.1 volt with reference to ground or reference voltage, ground or reference voltage also being the substrate voltage when the circuit is an integrated circuit. The voltage across the sense resistor 11 is amplified by a differential amplifier 15.
In low voltage systems such as, for example, automotive applications (about 12 volts), high current levels in about the 20 to 30 ampere range are common. When these currents are switched, as is common when utilizing pulse width modulation (PWM) control techniques, inductive dV/dT spikes and IR drops can cause voltage levels in high current paths to go above or below ground voltage or the voltage of the negative battery terminal. One such path is the load current monitoring path as shown in FIG. 1 where the offset voltage 13 is the result of the voltage spikes and IR drops. When the load is pulsed from the high voltage side of the load with recirculation taking place from the low voltage side of the load, there is a tendency for the voltage across the sense resistor to periodically go negative with reference to ground or the integrated circuit substrate. This presents a particular problem for a current sense amplifier since the amplifier must now be capable of operation with a common mode voltage below ground voltage or, in the case of an integrated circuit, below the substrate voltage. This is also true for a single transistor drive, as would be used in the case of a solenoid or a unidirectional motor, or other types of drive as well as for the exemplary H-bridge configuration as depicted in FIG. 1. Also, the amplifier must often operate from a single voltage supply and the output voltage is usually referenced to a less noisy analog ground voltage.
This problem has existed in bipolar implementations and has been addressed by use of Darlington topology, however these improvements in bipolar technology require additional components and therefore utilize additional die area in the case of integrated circuits, this being undesirable.